CBOR Web Token (CWT)Microsoftmbj@microsoft.comhttp://self-issued.info/Swedenerik@wahlstromstekniska.seSpotify ABBirger Jarlsgatan 61, 4trStockholm113 56Sweden+46702691499erdtman@spotify.comARM Ltd.6060Hall in TirolAustriaHannes.Tschofenig@arm.com
Security
ACE Working GroupInternet-DraftJSON Web TokenJWTClaimsCBORCOSEOAuthACE
CBOR Web Token (CWT) is a compact means of representing claims to be
transferred between two parties. CWT is a profile of the JSON Web
Token (JWT) that is optimized for constrained devices. The claims in a
CWT are encoded in the Concise Binary Object Representation (CBOR) and
CBOR Object Signing and Encryption (COSE) is used for added application
layer security protection. A claim is a piece of information asserted
about a subject and is represented as a name/value pair consisting of a
claim name and a claim value.
The JSON Web Token (JWT) is a standardized security token format
that has found use in OAuth 2.0 and OpenID Connect deployments, among other applications.
JWT uses JSON Web Signatures (JWS) and
JSON Web Encryption (JWE) to secure the contents of the JWT,
which is a set of claims represented in JSON .
The use of JSON for encoding information is popular for
Web and native applications, but it is considered inefficient for some
Internet of Things (IoT) systems that use low power radio technologies.
In this document an alternative encoding of claims is defined. Instead
of using JSON, as provided by JWTs, this specification uses
CBOR and calls this new structure "CBOR Web Token (CWT)", which is a
compact means of representing secured claims to be transferred between two
parties. CWT is closely related to JWT. It references the JWT claims
and both its name and pronunciation are derived from JWT. To protect the
claims contained in CWTs, the CBOR Object Signing and Encryption (COSE)
specification is used.
The suggested pronunciation of CWT is the same as the English word
"cot".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in "Key words for use in
RFCs to Indicate Requirement Levels" .
This document reuses terminology from JWT
and COSE .
The "Type3StringOrURI" term has the same meaning, syntax, and
processing rules as the "StringOrUri" term defined in Section 2
of JWT , except that Type3StringOrURI uses
CBOR major type 3 instead of a JSON string value.
The "Type6NumericDate" term has the same meaning, syntax, and
processing rules as the "NumericDate" term defined in Section 2
of JWT , except that Type6NumericDate uses
CBOR major type 6, with tag value 1, instead of a numeric JSON
value.
The key used to identify a claim value.
A CBOR map that contains the claims conveyed by the CWT.
The set of claims that a CWT must contain to be considered valid is
context dependent and is outside the scope of this specification.
Specific applications of CWTs will require implementations to
understand and process some claims in particular ways. However, in
the absence of such requirements, all claims that are not understood
by implementations MUST be ignored.
To keep CWTs as small as possible, the CBOR encoded claim keys are
represented using CBOR major type 0.
summarizes all keys
used to identify the claims defined in this document.
None of the claims defined below are intended to be
mandatory to use or implement. They rather provide
a starting point for a set of useful, interoperable claims.
Applications using CWTs should define which specific claims they
use and when they are required or optional.
The iss (issuer) claim has the same meaning, syntax, and
processing rules as the iss claim defined in Section 4.1.1
of JWT , except that the format MUST be a
Type3StringOrURI. The CBOR encoded claim key 1 MUST be used to
identify this claim.
The sub (subject) claim has the same meaning, syntax, and
processing rules as the sub claim defined in Section 4.1.2
of JWT , except that the format MUST be a
Type3StringOrURI. The CBOR encoded claim key 2 MUST be used to
identify this claim.
The aud (audience) claim has the same meaning, syntax, and
processing rules as the aud claim defined in Section 4.1.3
of JWT , except that the format MUST be a
Type3StringOrURI. The CBOR encoded claim key 3 MUST be used to
identify this claim.
The exp (expiration time) claim has the same meaning, syntax,
and processing rules as the exp claim defined in Section
4.1.4 of JWT , except that the format
MUST be a Type6NumericDate. The CBOR encoded claim key 4 MUST be
used to identify this claim.
The nbf (not before) claim has the same meaning, syntax,
and processing rules as the nbf claim defined in Section
4.1.5 of JWT , except that the format
MUST be a Type6NumericDate. The CBOR encoded claim key 5 MUST be
used to identify this claim.
The iat (issued at) claim has the same meaning, syntax,
and processing rules as the iat claim defined in Section
4.1.6 of JWT , except that the format
MUST be a Type6NumericDate. The CBOR encoded claim key 6 MUST be
used to identify this claim.
The cti (CWT ID) claim has the same meaning, syntax,
and processing rules as the jti claim defined in Section
4.1.7 of JWT , except that the format
MUST be of major type 2, binary string. The
CBOR encoded claim key 7 MUST be used to identify this claim.
How to determine that a CBOR data structure is a CWT is application-dependent.
In some cases, this information is known from the application context,
such as from the position of the CWT in a data structure at which the value must be a CWT.
One method of indicating that a CBOR object is a CWT is the use of
the "application/cwt" content type by a transport protocol.
This section defines the CWT CBOR tag as another means for applications to
declare that a CBOR data structure is a CWT.
Its use is optional, and is intended for use in cases in which
this information would not otherwise be known.
The CWT tag MUST prefix a tagged object using one of the COSE CBOR tags.
In this example, the COSE_Mac0 tag is used.
The actual COSE_Mac0 object has been excluded from this example.
To create a CWT, the following steps are performed. The order of the
steps is not significant in cases where there are no dependencies
between the inputs and outputs of the steps.
Create a CWT Claims Set containing the desired claims.
Let the Message be the binary representation of the CWT Claims Set.
Create a COSE Header containing the desired set of Header
Parameters. The COSE Header MUST be valid per the
specification.
Depending upon whether the CWT is signed, MACed, or encrypted,
there are three cases:
If the CWT is signed, create a COSE_Sign/COSE_Sign1 object
using the Message as the COSE_Sign/COSE_Sign1 Payload; all
steps specified in for
creating a COSE_Sign/COSE_Sign1 object MUST be followed.
Else, if the CWT is MACed, create a COSE_Mac/COSE_Mac0 object
using the Message as the COSE_Mac/COSE_Mac0 Payload; all
steps specified in for
creating a COSE_Mac/COSE_Mac0 object MUST be followed.
Else, if the CWT is a COSE_Encrypt/COSE_Encrypt0 object,
create a COSE_Encrypt/COSE_Encrypt0 using the Message as
the plaintext for the COSE_Encrypt/COSE_Encrypt0 object;
all steps specified in
for creating a COSE_Encrypt/COSE_Encrypt0 object MUST be
followed.
If a nested signing, MACing or encryption operation will be
performed, let the Message be the COSE_Sign/COSE_Sign1,
COSE_Mac/COSE_Mac0 or COSE_Encrypt/COSE_Encrypt0, and
return to Step 3, using a "content type" header value
corresponding to the media type "application/cwt" in the new
COSE Header created in that step.
Note: If integrity (signing/MACing) and confidentiality
(encryption) protection are needed, it is recommended to use an
authenticated encryption algorithm to save space and processing.
If needed by the application, add the appropriate COSE CBOR tag
to the COSE object to indicate type of COSE object.
If also needed by the application, add the CWT CBOR tag
to indicate that the COSE object is a CWT.
When validating a CWT, the following steps are performed. The order
of the steps is not significant in cases where there are no
dependencies between the inputs and outputs of the steps. If any of
the listed steps fail, then the CWT MUST be rejected -- that is,
treated by the application as an invalid input.
Verify that the CWT is a valid CBOR object.
If the object begins with the CWT CBOR tag, remove it
and verify that one of the COSE CBOR tags follows it.
If the object is tagged with one of the COSE CBOR tags, remove it
and verify that it corresponds to the structure of the following COSE object.
Verify that the resulting COSE Header includes only parameters
and values whose syntax and semantics are both understood and
supported or that are specified as being ignored when not
understood.
Use the CBOR tag to determine the type of the CWT,
COSE_Sign/COSE_Sign1, COSE_Mac/COSE_Mac0, or
COSE_Encrypt/COSE_Encrypt0.
Depending upon whether the CWT is a signed, MACed, or encrypted,
there are three cases:
If the CWT is a COSE_Sign/COSE_Sign1, follow the steps
specified in Section 4
(Signing Objects) for validating a COSE_Sign/COSE_Sign1
object. Let the Message be the COSE_Sign/COSE_Sign1 payload.
Else, if the CWT is a COSE_Mac/COSE_Mac0, follow the steps
specified in Section 6
(MAC Objects) for validating a COSE_Mac/COSE_Mac0 object.
Let the Message be the COSE_Mac/COSE_Mac0 payload.
Else, if the CWT is a COSE_Encrypt/COSE_Encrypt0 object,
follow the steps specified in
Section 5 (Encryption
Objects) for validating a COSE_Encrypt/COSE_Encrypt0 object.
Let the Message be the resulting plaintext.
If the COSE Header contains a "content type" header value
corresponding to the media type "application/cwt", then
the Message is a CWT that was the subject of nested signing
or encryption operations. In this case, return to Step 1,
using the Message as the CWT.
Verify that the Message is a valid CBOR object; let the CWT Claims Set be this CBOR object.
The security of the CWT is dependent on the protections offered by COSE.
Unless the claims in a CWT are protected, an adversary can modify, add, or remove claims.
Since the claims conveyed in a CWT may be used to make authorization decisions,
it is not only important to protect the CWT in transit but also to ensure that
the recipient can authenticate the party that assembled the claims and created the CWT.
Without trust of the recipient in the party that created the CWT, no sensible authorization decision can be made.
Furthermore, the creator of the CWT needs to carefully evaluate each claim value prior to including it in the CWT
so that the recipient can be assured of the validity of the information provided.
This section establishes the
IANA "CBOR Web Token (CWT) Claims" registry.
Values are registered on a Specification Required
basis, on the advice of one or more Designated Experts.
However, to allow for the
allocation of values prior to publication, the Designated Experts may approve
registration once they are satisfied that such a specification will be published.
Criteria that should be applied by the Designated Experts includes
determining whether the proposed registration duplicates existing functionality,
whether it is likely to be of general applicability
or whether it is useful only for a single application,
and whether the registration description is clear.
The human-readable name requested (e.g., "iss").
Brief description of the claim (e.g., "Issuer").
Claim Name of the equivalent JWT claim as registered in .
CWT claims should normally have a corresponding JWT claim.
If a corresponding JWT claim would not make sense,
the Designated Experts can choose to accept registrations
for which the JWT Claim Name is listed as "N/A".
Key value for the claim. The key value MUST be an integer in the range of 1 to 65536.
CBOR major type and optional tag for the claim.
For Standards Track RFCs, list the "IESG". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included.
Reference to the document or documents that specify the parameter, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.
Claim Name: issClaim Description: IssuerJWT Claim Name: issCBOR Key Value: 1CBOR Major Type: 3Change Controller: IESGSpecification Document(s): of [[ this specification ]]Claim Name: subClaim Description: SubjectJWT Claim Name: subCBOR Key Value: 2CBOR Major Type: 3Change Controller: IESGSpecification Document(s): of [[ this specification ]]Claim Name: audClaim Description: AudienceJWT Claim Name: audCBOR Key Value: 3CBOR Major Type: 3Change Controller: IESGSpecification Document(s): of [[ this specification ]]Claim Name: expClaim Description: Expiration TimeJWT Claim Name: expCBOR Key Value: 4CBOR Major Type: 6, tag value 1Change Controller: IESGSpecification Document(s): of [[ this specification ]]Claim Name: nbfClaim Description: Not BeforeJWT Claim Name: nbfCBOR Key Value: 5CBOR Major Type: 6, tag value 1Change Controller: IESGSpecification Document(s): of [[ this specification ]]Claim Name: iatClaim Description: Issued AtJWT Claim Name: iatCBOR Key Value: 6CBOR Major Type: 6, tag value 1Change Controller: IESGSpecification Document(s): of [[ this specification ]]Claim Name: ctiClaim Description: CWT IDJWT Claim Name: jtiCBOR Key Value: 7CBOR Major Type: 2Change Controller: IESGSpecification Document(s): of [[ this specification ]]
This section registers the application/cwt media type
in the "Media Types" registry
in the manner described in RFC 6838,
which can be used to indicate that the content is a CWT.
Type name: application
Subtype name: cwt
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section of [[ this specification ]]
Interoperability considerations: N/A
Published specification: [[ this specification ]]
Applications that use this media type:
IoT applications sending security tokens over HTTP(S) and other transports.
Fragment identifier considerations: N/A
Additional information:Magic number(s): N/AFile extension(s): N/AMacintosh file type code(s): N/A
Person & email address to contact for further information:
IESG, iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: none
Author: Michael B. Jones, mbj@microsoft.com
Change controller: IESG
Provisional registration? No
This section registers the CoAP Content-Format ID for the "application/cwt" media type
in the "CoAP Content-Formats" registry
established by .
Media Type: application/cwtEncoding: - Id: TBD (maybe 61) Reference: [[ this specification ]]
This section registers the CWT CBOR tag
in the "CBOR Tags" registry
established by .
CBOR Tag: TBD (maybe 61 to use the same value as the Content-Format)Data Item: CBOR Web Token (CWT)Semantics: CBOR Web Token (CWT), as defined in [[ this specification ]]Reference: [[ this specification ]]Point of Contact: Michael B. Jones, mbj@microsoft.comJSON Web Token ClaimsIANAMedia TypesIANACoAP Content-FormatsIANAConcise Binary Object Representation (CBOR) TagsIANA
This appendix includes a set of CWT examples that show how the CWT Claims Set can be protected.
There are examples that are signed, MACed, encrypted, and that use nested signing and encryption.
To make the examples easier to read, they are presented both as hex strings and
in the extended CBOR diagnostic notation .
The CWT Claims Set used for the different examples
displays usage of all the defined claims. For signed and MACed
examples, the CWT Claims Set is the CBOR encoding as a binary string.
This section contains the keys used to sign, MAC, and encrypt the messages in this appendix.
Line breaks are for display purposes only.
This section shows a signed CWT with a single recipient and a full CWT Claims Set.
The signature is generated using the private ECDSA key from
and it can be validated using the public part of the ECDSA key from .
Line breaks are for display purposes only.
This section shows a MACed CWT with a single recipient and a full CWT Claims Set.
The MAC is generated using the 256-bit symmetric key from with a 64-bit truncation.
Line breaks are for display purposes only.
This section shows an encrypted CWT with a single recipient and a full CWT Claims Set.
The encryption is done with AES-CCM mode using the 128-bit symmetric key from with a 64-bit tag and 13-byte nonce, i.e., COSE AES-CCM-16-64-128.
Line breaks are for display purposes only.
This section shows a Nested CWT, signed and then encrypted, with a single recipient and a full CWT Claims Set.
The signature is generated using the private ECDSA key from
and it can be validated using the public ECDSA parts from .
The encryption is done with AES-CCM mode using the 128-bit symmetric key from
with a 64-bit tag and 13-byte nonce, i.e., COSE AES-CCM-16-64-128.
The content type is set to CWT to indicate that there are multiple
layers of COSE protection before finding the CWT Claims Set. The decrypted
ciphertext will be a COSE_sign1 structure. In this example, it is the same
one as in , i.e., a Signed CWT Claims Set.
Note that there is no limitation to the number of layers; this is an
example with two layers.
Line breaks are for display purposes only.
This specification is based on JSON Web Token (JWT) ,
the authors of which also include Nat Sakimura and John Bradley.
Ludwig Seitz and Göran Selander have made contributions the specification.
[[ to be removed by the RFC Editor before publication as an RFC ]]
-03
Reworked the examples to include signed, MACed, encrypted, and nested CWTs.
Defined the CWT CBOR tag and explained its usage.
-02
Added IANA registration for the application/cwt media type.
Clarified the nested CWT language.
Corrected nits identified by Ludwig Seitz.
-01
Added IANA registration for CWT Claims.
Added IANA registration for the application/cwt CoAP content-format type.
Added Samuel Erdtman as an editor.
Changed Erik's e-mail address.
-00
Created the initial working group version based on draft-wahlstroem-ace-cbor-web-token-00.